Labyrinth type rotary seals generally comprise two relatively rotatable members. One such member typically includes one or more circumferential teeth which are coaxially positioned with respect to a circumferential sealing surface on the second member. Seals of this type are used to restrict fluid flow between cavities formed by stationary and rotating members without impeding the rotational movement of the rotating member.
A disadvantage of seals of this type occurs when the temperature of the fluid leaking through the seal undergoes a rapid change, such as during transient operating conditions. Typically, one member responds quickly to the change in temperature resulting in thermally induced radial growth. At the same time, the other member heats more slowly thereby thermally growing at a slower rate. Many factors may contribute to the different rates of thermal response of the two members. For example, an abradable shroud on the flow path surface of the stationary member may utilize a material with a lower coefficient of thermal conductivity than the material of the rotating member.
At equilibrium operating temperature, the separation or gap between rotating and stationary members is designed to be fixed at a typically small value. However, even at equilibrium, this gap can vary with temperature changes on the back side of the stationary seal.
During transient conditions when the fluid is changing temperature rapidly, the differential growth between rotating and stationary members may result in a rub therebetween. Such rubs result in the wearing down of the stationary seal surface or circumferential teeth which increase clearance at steady state operation. Increased clearance degrades seal performance.